1. Field of the Invention
The present invention relates to semiconductor manufacturing, and, in particular, to a technique for reducing dambar burrs on semiconductor devices.
2. Description of the Related Art
Leads for surface-mount semiconductor devices are fabricated in a frame with a portionxe2x80x94known as a dambarxe2x80x94that must be subsequently removed. FIG. 1 shows a cutting die for dedam/dejunk die set, which is typically used to remove the dambar. As shown in FIG. 1, a dedam/dejunk die set generally comprises a cutting insert 102, mounted, with a mounting screw 104, within a pocket 106 of a holding plate 108, and a punch (not shown). The cutting insert has a cutting edge 110 and a screw clearance 112 in its edge opposite the cutting edge. Holding plate 108 has a screw clearance 114 surrounding a threaded hole 116.
After placing cutting insert 102 within pocket 106 of holding plate 108, mounting screw 104 is inserted through screw clearances 112 and 114 and fastened into threaded hole 116 in mounting plate 108 to secure the cutting insert in place. In particular, the head of mounting screw 104 presses against the bottom of screw clearance 112 in cutting insert 102, restraining movement of cutting insert 102. To sever the dambar from each lead, the dambars are pinched between cutting edge 110 of cutting insert 102 and the punch.
The clearance between the cutting edge of the cutting insert and the punch is preferably between about 5% and 9% of the lead frame thickness (e.g., about 0.008 mm in one embodiment). When the clearance between the cutting insert and the punch is too great, a small portion of the lead frame may wrap around the cutting insert before the lead frame is severed, causing a dambar burr. Dambar burrs can cause alignment and soldering reliability problems when the surface-mount semiconductor device is installed. Removing dambar burrs requires additional processing and inspection time, increasing cost and reducing process throughput.
Referring again to FIG. 1, over a period of use, pocket 106 in holding plate 108 can become worn, causing an increased clearance between cutting insert 102 and holding plate 108. In that case, the clockwise tightening action of mounting screw 104 restraining cutting insert 102 relative to holding plate 108 can cause the cutting insert to shift laterally to the right, as shown in the top view of FIG. 2. This shift of the cutting insert can cause an increase in the clearance between the cutting insert and the punch, increasing the likelihood of dambar burrs.
The present invention provides a technique for reducing dambar burrs formed by a die set having a cutting insert fastened by a mounting screw into the pocket of a holding plate. According to the present invention, the pocket and the cutting insert are configured to receive a clamping plate that inhibits the cutting insert from moving when the cutting insert is secured within the pocket of the holding plate by a mounting screw. In one implementation, the shape of the clamping plate is defined by two intersecting circular arcs of different diameter, where the smaller arc matches the screw clearance in the cutting insert, and the larger arc matches the pocket of the holding plate. The clamping plate is fastened to the holding plate with the smaller arc abutting the screw clearance in the cutting insert and the larger arc abutting the holding plate pocket. In this way, the clamping plate inhibits movement of the cutting insert, when the mounting screw is tightened.
In one embodiment, the present invention is an apparatus for mounting a cutting insert of a die set, wherein the cutting insert has a screw clearance, comprising (a) a holding plate defining a pocket configured to receive the cutting insert; and (b) a clamping plate configured to fit within the pocket of the holding plate and within the screw clearance of the cutting insert, after the cutting insert is inserted into the pocket of the holding plate, to inhibit lateral motion of the cutting insert within the pocket of the holding plate when a mounting screw is inserted through a through hole in the clamping plate and tightened into a corresponding threaded hole in the holding plate.
In another embodiment, the present invention is a method for cutting dambars from semiconductor devices, comprising the steps of (a) placing a semiconductor device having one or more dambars into a die set; and (b) cutting the one or more dambars with a cutting edge of a cutting insert of the die set, wherein the cutting insert is secured within a pocket of a holding plate of the die set using a mounting screw and a clamping plate that inhibits lateral motion of the cutting insert within the pocket when the mounting screw is tightened within a threaded hole in the holding plate.